The invention relates to a method of treating a sample of an alloy which is capable of transforming between martensitic and austenitic phases, to render the alloy pseudoelastic.
Alloys which are capable of transforming between martensitic and austenitic phases are generally able to exhibit a shape memory effect. The transformation between phases may be caused by a change in temperature: for example, a shape memory alloy in the martensitic phase will begin to transform to the austenitic phase when its temperature increases to a temperature greater than A.sub.s, and the transformation will be complete when the temperature is greater than A.sub.f. The reverse transformation will begin when the temperature of the alloy is decreased to a temperature less than M.sub.s, and will be complete when the temperature is less than M.sub.f. The temperatures M.sub.s, M.sub.f, A.sub.s and A.sub.f define the thermal transformation hysteresis loop of a shape memory alloy. Commonly known alloys which are capable of transforming in this way are based on nickel-titanium, for example as disclosed in U.S. Pat. No. 3,753,700, U.S. Pat. No. 4,505,767 and U.S. Pat. No. 4,565,589, or on copper, for example as disclosed in U.S. Pat. No. 4,144,057 and U.S. Pat. No. 4,144,104.
It has been found that, under certain conditions, shape memory alloys are capable of being deformed elastically beyond what would normally be expected to be the elastic limit of metallic materials. This phenomenon is referred to as pseudoelasticity. As discussed in the paper presented by T. W. Duerig and G. R. Zadno at the International meeting of the Materials Research Society which took place in Tokyo in June 1988, certain alloys are capable of exhibiting pseudoelasticity of two types. The present invention is concerned with "non-linear pseudoelasticity" which arises in appropriately treated alloys while they are in their austenitic phase at a temperature which is greater than M.sub.s and less than M.sub.d, where M.sub.d is the maximum temperature at which the transformation to the martensitic phase can be induced by the application of stress. An article formed from an alloy which exhibits non-linear pseudoelasticity can be deformed substantially reversibly by 8% or more. Non-linear pseudoelasticity is a shape memory effect involving transformation between martensitic and austenitic phaser of a shape memory alloy, but it need not involve a change in the temperature of the alloy. In contrast, "linear pseudoelasticity" is believed not to be accompanied by a phase change. An article formed from an alloy which exhibits linear pseudoelasticity can be deformed substantially only reversibly by about 4%.
Thus non-linear pseudoelasticity has the advantage that an article formed from an alloy which exhibits it can be deformed by significantly more than one formed from an alloy which exhibits linear pseudoelasticity.
The process which is generally used to confer non-linear pseudoelastic properties on a shape memory alloy involves annealing the alloy at a temperature above that at which the alloy has recovered to a significant degree but below that at which the alloy is fully recrystallized. For example, a sample of a shape memory alloy may be formed into a wire by a conventional cold-drawing technique. It may then be rendered pseudoelastic by annealing. As a result of the annealing step, the pseudoelastic properties of the alloy are characterized: for present purposes, reference will be made to the maximum elastic strain, which term denotes the strain which is imparted to a sample at the elastic limit and can be recovered substantially elastically, and to the effective elastic modulus, which term denotes the ratio of the applied stress to the imparted strain.
If it is desired to provide a sample in a configuration which is different from that in which it is drawn or otherwise formed, it is deformed and held in the deformed configuration during the annealing step. The deformation of the sample may take place before the annealing step is started or during the annealing step, the annealing step ensuring that the deformed sample has pseudoelastic properties. The configuration of a sample which has been annealed may be changed by heating the sample again to the annealing temperature and holding it in the deformed configuration at that temperature.
It is inconvenient to deform a sample of a shape memory alloy while it is annealed since, for example, it renders continuous annealing of a shape memory wire difficult and impractical. However, it is known that if the sample is deformed so as to change its configuration after it has cooled after the annealing step, the pseudoelastic properties of the alloy characterized as a result of the annealing step are affected adversely and, in some cases, destroyed.